Dysfunction of p53 is associated with over half of all cases of human cancer. There is growing evidence that changes that occur in cancer cells trigger the inhibitory activity of p53, thereby imposing a strong pressure for elimination of p53 function in order for the tumor to grow further and become more aggressive. Understanding the mechanisms that regulate p53 function may provide important clues towards the restoration of that function in tumor cells where it is missing altogether and towards its enhancement in cases when it is partly attenuated. The proposed project will explore several pathways that modulate cellular p53 activity. Much of the regulation of p53 is through Mdm2, a protein that binds p53 and inactivates it. In parallel to looking at p53, the experiments will also monitor changes that occur in Mdm2 in response to signals. The impact of these changes on the p53-inhibitory effect of Mdm2 will be assessed. Mdm2 becomes phosphorylated in response to DNA damage, and this proceeds p53 activation. Mdm2 controls the ubiquitination and proteolytic degradation of p53. Following identification of the site(s) on Mdm2 modified upon DNA damage, these sites will be mutated. Mutant proteins will be assayed for p53-inhibitory activities in transfected cells and in a cell-free assay for p53 ubiquitination. P53 can become upregulated by excess activity of 2 important proteins: B-catenin, whose deregulation leads to colorectal and other cancers, and WRN, whose dysfunction leads to a premature aging syndrome. The biochemical mechanisms leading to p53 upregulation in each case will be explored. Animal models will be employed to test the hypothesis that B-catenin deregulation imposes a selective pressure for p53 inactivation. The consequences of the WRN-p53 interaction will be evaluated by transfection experiments using cultured human and mouse cells with defective WRN function. P53 binds histone acetylases and becomes acetylated in response to DNA damage. The impact of acetylation on p53 function will be evaluated through the generation of appropriate p53 point mutants and their analysis in a panel of biochemical and biological assays. The possibility that p53 is also targeted by deacetylases, which reverse the action of acetylases, will be evaluated by monitoring direct protein-protein interactions and changes in p53 acetylation. Mdm2 acetylation will be evaluated as an additional mechanism for modulating p53. Nitric oxide (NO) is an important signal tansducer molecular. NO ca upregulate p53. Underlying biochemical mechanisms will be studied. Finally, p53 activity will be monitored in vivo in transgenic reporter mice subjected to ischemia. P53 may play a role as a mediator of tissue damage in this clinically important condition.